The foreign secretary and the Treasury have written confidential letters to cabinet colleagues strongly condemning the plans drawn up by David Blunkett, the home secretary.

Both letters were sent a few days before Blair backed Blunkett’s ID card scheme in his speech to the party conference in Bournemouth.

Under Blunkett’s plans, disclosed by The Sunday Times in July, British residents would not be required to carry a card at all times but would have to produce it within a few days if requested.

In an uncompromising six-page attack on the scheme, Straw says the plans are “flawed” and warns: “The potential for a large-scale debacle which harms the government is great.”

The documents also reveal that the cards would be free for asylum seekers, although British citizens would have to pay £40, and even those on state benefits or retired but under 75 would face a reduced fee of £5.

A five-page letter from the Treasury hints at a huge political backlash because it would have to be treated as a tax rise at a time when the government is sensitive on the issue.

It argues that by forcing people to pay £40 for the compulsory card the Office for National Statistics would categorise it as a tax, and adds that objections “raised by cabinet are substantial”.

Although there have been rumblings of cabinet discontent, the documents reveal for the first time the depth of the divisions over ID cards, making it the most sensitive political issue since the cabinet row over the euro.

It is now highly unlikely the scheme will make it into next month’s Queen’s Speech, with Blunkett acknowledging that he will have to shelve the plan unless he can overturn opposition in the cabinet.

The row has again pitted Blairite reformers against Brown’s supporters and raised suspicions that cabinet ministers are taking sides. Straw is said by colleagues to have formed a new alliance in a bid to hedge his bets in case the chancellor eventually succeeds Blair as prime minister.

Blairites including John Reid, the health secretary, Geoff Hoon, the defence secretary, and Charles Clarke, the education secretary, are understood to be in favour of the ID card scheme. But John Prescott, the deputy prime minister, Alistair Darling, the transport secretary, and Patricia Hewitt, the trade secretary, are against.

So far only Hewitt has gone public on her objections to the scheme, saying that ID cards raise “enormous questions” of principle and practicality.

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